Although whip or aerial antennas have been used on motor vehicles because they offer very good reception for a mobile receiver, they suffer the disadvantages that they provoke wind noise and aerodynamic drag, they interfere with or detract from the styling of the vehicle body and they are exposed to mechanical damage. Accordingly, attempts have been made to conceal the antennas since the mid-1970s. However, previous designs for example, antennas mounted in the front glass, were discontinued because previously known designs suffered from poor reception performance under weak field strength conditions and due to their high susceptibility to pick up vehicle electrical noise.
Some improvements have been made in antenna design in order to overcome the previously identified problems. For example, slot antennas have exhibited improved performance over previously known embedded conductor antennas. However, slot antennas are made with a large surface of conductive material in which the slot is formed. However, large body panels that contain such conductive surfaces are often not well aligned for optimum signal reception. Moreover, when considering the substantial metallic content of most vehicle structures, the dimensions of the passenger compartment comprise an electrically small enclosure compared with the wave length of the frequencies in an AM broadcast band. Such a structure significantly attenuates the signal, and thus skilled designers would be conventionally encouraged to conceal an AM antenna at a location outside the cabin enclosure.
Antennas formed by conductors located in or on the vehicle glass is another known implementation of a concealed antenna system. Conducting lines or surface strips, for example, contained in the rear glass assembly, may provide sufficient gain for good AM and FM reception. For example, a heater grid structure that exists on many vehicles for window defogging or defrosting has been used as an AM antenna. However, as the effective antenna length is increased with this type of design, the antenna source capacitance increases. Additionally, the close proximity to sheet metal body panels results in an increased shunt capacitance to ground. In addition, any capacitance imposed by a cable connecting the antenna to a radio signal receiver in the motor vehicle will add substantially to the shunt capacitance and thus substantially reduce the reception performance between the antenna and the receiver.
In addition, the proximity of antennas to noisy vehicle wiring and ground loops, as noise currents often flow in the sheet metal as part of the ground return for many different circuits, can result in a coupling of unwanted noise into the antenna system. Accordingly, careful grounding, wire routing and EMC suppression procedures would appear to be necessary to effectively divert noise currents away from the antenna circuit. As a result, the additional features to implement these considerations substantially increase the complexity and expense of incorporating such antennas in motor vehicles.
Low noise broad band preamplifiers have been used to compensate for reduced antenna gain and improve weak signal reception. While such amplifiers increase the signal level delivered to the receiver, the improvement in the signal to noise ratio perceived by a listener may be minimal due to an ambient RF noise level near the vehicle. In addition, preamplifiers may degrade the dynamic range of the receiver system resulting in the increased incidence of overload, inter-modulation and cross-modulation distortion. This problem is particularly acute during reception of extremely strong transmission due to for example close proximity to signal source transmitter. Although the added preamplifier gain will improve overall system gain, the ambient noise level must be sufficiently low in order to realize an improvement in the signal to noise ratio. Thus, the theoretical improvement sought to be obtained by the radio frequency preamplifier may be made ineffective by man made, natural and atmospheric noise often encountered. In addition, an additional amplifier substantially increases the cost and complexity of the interface circuit between the antenna and the radio receiver.